I have built many regenerative vacuum tube shortwave receivers, now I wanted to attempt to build a simple shortwave transmitter.

The Inspiration

While searching on the Internet, I came across an article from "The Heart of Texas DX Society" about building a simple one tube transmitter using a common 6L6 Power Beam Tetrode Tube. This got my creative juices flowing and I decided this will be my first vacuum tube transmitter project!

The 6L6 Transmitter I am to build is that of CW (Continuous Wave) type. A CW transmitter conveys intelligence in the form of Morse Code dots and dashes by turning on and off the transmitter. It does not convey the spoken word using AM (Amplitude) or FM (Frequency) modulation. CW is the oldest way of conveying information by wireless.

The 6L6 Transmitter is basically a crystal controlled radio frequency oscillator. C1, Coil, and C2 form a tuned matching circuit used to match the high impedance of the oscillator circuit to the low impedance of the antenna. The 2.5 MH choke connected in series between the 300 Volt DC Power Supply and the Plate (pin 3) of 6L6 prevents RF from feeding back into the power supply. The Crystal and 2.5MH choke connected in parallel connected to the grid (pin 5) forms a frequency stable oscillator circuit that is amplified by the 6L6 tube. This oscillator circuit is controlled by the Key switch, which opens and closes the path to ground for the 6L6 tube amplifier circuit thereby stopping and starting the oscillation and RF output. The 220 ohm 1Watt resistor in series with the cathode (pin 8) acts as a current limiting resistor.

The crystal and coil can be changed in the circuit so that the transmitter can be used on the 40 meter or 80 meter Amateur radio bands.

At the time of the writing I only had a 6V6 tube and power output was only 5 Watts. I expect a power output of at least 25 Watts once I replace the 6V6 tube with a 6L6!

Below is a reprint of the schematic from "The Heart of Texas DX Society" web site article written by James Tobola - KC5LDO

Step 1 Enclosure Build

I chose a classic "L" design for the enclosure. It is made from pine I salvaged from a shelf in the garage. It took quite a bit of effort to remove the oil and grease stains. I then used a palm sander to smooth out the imperfections. The front is held in place by three wood screws.

Step 2 Front Panel Parts Placement

My next step was to determine where to place the controls on the front panel. I placed the Plate and Load variable capacitors next to each other as it is important to keep wire connections in the RF section as short as possible so as not to introduce stray inductance into the mix.

Step 3 Drilling the Front Panel

The shafts of the variable capacitors, selector switch, and key jack, are too short to connect through the 3/4 width of the front panel. I used a Wooden Hole Saw Set to drill concentric holes 1/2 deep into the back of the front panel so that the shafts would protrude from the front.

Step 4 Bottom Panel Parts Placement

I created a paper template that showed parts placement and mounting holes. I then used a metal punch to transfer the hole placement to the top of the bottom panel. In addition, I including wiring information on paper template.

Wood Putty Saves the Day!

Four of the holes used to mount the standoffs are counter-sunk. I accidentally made the holes the larger diameter of the screw head. This is were a little wood putty saved the day!

Step 5 Dry Fit Parts

I mount all of the parts to the wooden chassis and check the fit before applying a protective layer of Polyurethane.

Step 6 Winding the Coils

The 6L6 Transmitter uses two interchangeable coils mounted to a tube base. One coil is for 40M band operation, the other is for the 80M band.

Step 7 Mounting the Crystal

I also mount the appropriate crystal in the tube base so switching bands is a one step operation.

Step 8 Final Finish

I apply two coats of Polyurethane followed by sanding with a fine grit sandpaper. I then apply a final coat.

Step 9 Apply Foil Coating

I had a lot of problems with hand capacitance modifying the operation of the regenerative radios I built. As such, I glue aluminum wrap to the back side of the front panel and tie to ground in order to hopefully reduce this issue.

Step 10 Final Assembly

I can now permanently mount the parts to the wooden chassis, now that the Polyurethane coating has been applied and the foil backing has been applied to the front panel.

Step 11 Installing Mounting Feet

I install mounting feet to the corners of the bottom of the bottom panel. This prevents the bottom from getting scuffed up when being moved. How many times can I use the word bottom in a sentence ;-)

Step 12 Filament Wiring

I wire the connections to the tube filament first.

Step 13 Ground Wiring

Next I wire all connections tied to ground.

Step 14 All Other Wiring

Then I wire everything else!

Step 15 Install Vacuum Tube and Coil, label connections

The next step is to install the vacuum tube and the coil.

Step 16 Install knobs, label front panel

The final step in the assembly process is to install the knobs and label the front panel.

Step 17 Initial Testing

I use an Heathkit HP-23A to provide the 6.3 Volts AC filament and 300 Volts DC required for operation of the 6L6 Transmitter. I had to open up the HP-23A and jumper to the proper Voltage connections as I do not have the power output cable that comes with this power supply.I also connected I Digital Multimeter in order to monitor the voltages during operation.

The 6L6 Tube Transmitter in Operation!

See my 6L6 CW Vacuum Tube Transmitter in operation. Please note that I did not have a 6L6 Vacuum tube on hand and used a 6V6 tube instead. As such, the transmitter power output was only about 5 Watts.

Conclusion

This was a most gratifying project. When not in use it is proudly displayed on a shelf in my basement "Man Cave". Building a 6L6 CW Transmitter is a great way to learn about the electronics of the past.